The comparison group in a trial that doesn't receive the intervention being tested.
In a controlled trial, the control arm is the group of participants who receive a placebo, usual care, sham treatment, or no intervention, providing a baseline against which the experimental arm's outcomes are compared. This isolates the effect attributable to the intervention itself from natural history, regression to the mean, and non-specific effects such as attention or expectation. What counts as an appropriate control (e.g., placebo vs. active comparator vs. waitlist) varies by question and can materially affect how findings should be interpreted.
This guide was auto-drafted and is pending editorial review.